This proposal is a new direction for my laboratory, which is to define developmental origins of sex differences in susceptibility to multiple sclerosis (MS). MS is an autoimmune, demyelinating disease of the central nervous system (CNS) that has a strong sex bias, with the female to male ratio currently approaching 4:1. Relapsing- remitting MS RRMS, the most common form of the disease in women, is a condition in which recurrent episodes of new neurological dysfunction (relapses) are separated by periods of clinical stability (remission). The SJL strain of mice is a standard model for examining sex differences in MS, as it exhibits a similar sex bias in susceptibility and RREAE in female animals. In published studies we showed that cell intrinsic sex differences in the expression of the sphingosine 1-phosphate receptor 2 (S1PR2) on central nervous system (CNS) vasculature of SJL mice regulates endothelial cell polarity by destabilizing adherens junctions and contributes to the development of disease cycles in females compared with males. Not yet defined is how the process of normal sexual differentiation patterns activity in pathways that regulate blood-brain barrier (BBB) stability. In this proposal we will use sex differences in S1PR2 expression to develop approaches that will uncover molecular mechanisms that regulate sex differences in BBB biology. Because evidence suggests that acute effects of estrogens are not responsible for sex differences in RRMS and in S1PR2 expression, the focus of this proposal is the role of sex chromosome complement and the organizational effects of sex hormones on sex differences in S1PR2 expression at the BBB. We hypothesize that sex differences in endothelial cell expression of S1PR2 occur via organizational effects that occur during development. We further hypothesize that chromatin regulation establishes additional sex-specific effects through altered expression of genes involved in BBB function. In this proposal we will: 1) Determine whether sex-differences in S1PR2 expression occur via cell intrinsic endothelial cell alterations in gene expression; and 2) Determine whether sex chromosome complement and/or organizational effects of sex hormones underlie sex differences in BMEC S1PR2 expression. Our findings will define new interfaces between sexually dimorphic gene expression and BBB function, and may identify new strategies to treat disease in MS patients in a sex-specific fashion.